There are two major types of infectious vaccinia virus particles: mature virions (MVs) and enveloped virions (EVs). The MVs, which consist of a DNA-protein core surrounded by a lipoprotein membrane, are assembled in cytoplasmic viral factories and contain about 80 polypeptides. A subpopulation of MVs is wrapped by modified trans-Golgi or endosomal cisterna containing additional viral membrane proteins, transported along microtubules to the cell periphery, and exits the cell through the plasma membrane. The EVs are essentially MVs with an additional membrane that is disrupted prior to fusion of the MV with the cell during entry. The first step in virus replication is entry. The mechanisms used by poxviruses are complicated by the existence of two distinct infectious forms with different external membranes. Another complicating factor is the ability of vaccinia virus to enter cells through the plasma membrane and by endocytosis. We have identified a complex comprised of at least 11 proteins that are conserved in all poxviruses and required for entry of both infectious forms of virus. These proteins are conserved in all members of the poxvirus family, suggesting a common entry mechanism. One of the most challenging questions is how the initial viral membrane of the immature virion is formed. Using biochemical, genetic and microscopic approaches, we have obtained evidence that the endoplasmic reticulum is the source of the viral membrane and have identified several viral proteins involved in this process. In addition, we have shown that retrograde transfer is required for the formation of the wrapping membrane.